The field of the present invention is four cycle engines having cylinders of noncircular cross section.
Engines have been developed which employ cylinders of noncircular cross section. Such engines which have an oblong cross section can increase the inlet and outlet port areas relative to the cross-sectional area of the cylinder over that which is possible with cylinders of circular cross section. Valve arrangements have been devised for such engines to increase aspiration efficiency. One such engine is illustrated in U.S. Pat. No. 4,256,068 issued to Shoichiro Irimajiri et al and entitled OBLONG PISTON AND CYLINDER FOR INTERNAL COMBUSTION ENGINE.
Such existing four cycle internal combustion engines whose cylinders are not circular in cross section have been devised in accordance with the shapes illustrated in FIGS. 1, 2 and 3. In FIG. 1, the cylinder H is shown to be two semicircular sections connected by two straight segments. The semicircular sections have the radius r and the straight sections extend between points P.sub.1. FIG. 2 illustrates another embodiment of a cylinder H having circular segments S.sub.1 of short radius r.sub.1 and circular segments S.sub.2 of long radius r.sub.2. The segments are connected at points P.sub.2. Engine cylinders, as illustrated in FIGS. 1 and 2, constructed of distinct differently curved segments require points of curvature discontinuity such as found at P.sub.1 and P.sub.2. With such discontinuities, a cutter employed in the forming of the surfaces of such cylinders is unable to smoothly traverse these points. As a result, high accuracy cannot be obtained, excessive time is required for the processing of the cylinder and the cutter experiences early wear. Thus, mass production becomes difficult although engines conforming to the cylinder designs of FIGS. 1 and 2 can improve gas flow efficiency and can be made using limited production techniques.
A further cylinder H which has been previously contemplated for cylinders of noncircular cross section is illustrated in FIG. 3. FIG. 3 has a true elliptical form. This form is more amenable to mass production techniques. As there is no curvature discontinuity, high accuracy, reduced processing time and longer cutter life may be realized. However, such a true ellipse creates areas D at either end of the cylinder which are narrowed considerably compared to the midsection of the cylinder. Dead spaces occur in this area as there is insufficient room for valve placement. Furthermore, the end portions of the cylinder are so curved that it becomes difficult to prepare and assemble a ring on a conforming piston in these areas.
Piston rings for such cylinders having noncircular cross sections have been devised. One such type of ring is the "expansion type" which is pressed outwardly against the inner wall of the cylinder by a device fitted between the piston and the piston ring. One such device is illustrated in U.S. Pat. No. 4,362,135 to Shoichiro Irimajiri, entitled PISTON RING OF INTERNAL COMBUSTION ENGINE. Another type of piston ring which has been devised for such cylinders is the self tension type which is pressed against the inner wall of the cylinder by means of its own tensile strength with the relaxed position of the ring being larger than the cylinder within which it is compressed. One such ring for a noncircular cylinder is disclosed in U.S. Pat. No. 4,198,065 to Takeo Fujui entitled PISTON RING FOR INTERNAL COMBUSTION ENGINE. The self tensioning type of piston ring tends to be more widely used as it has more advantages in terms of better sealing quality and cost.
As mentioned above, certain problems may accompany the fabrication and installation of such piston rings on pistons designed to conform to noncircular cylinders. With each of the cross-sectional shapes of cylinders illustrated in FIGS. 1 and 2, the abrupt or discontinuous change in curvature at either points P.sub.1 or P.sub.2 also required of the piston ring can result in stress concentrations in use. Fabrication of such curves may also be more difficult and, where straight sections are employed, they preferably include inwardly curved configurations in the relaxed state to overcome bending loads when positioned in the cylinder. Maintaining accuracy in the fabrication of such complex curves becomes difficult.
Consequently, the fabrication and assembly of components for engines having noncircular cylinders as illustrated in FIGS. 1 and 2 can be difficult. The configuration of FIG. 3 overcomes certain of the fabrication problems encountered with the configurations of FIGS. 1 and 2. However, ring assembly with the piston may be difficult and dead spaces can occur at the narrowed ends of the elliptical cylinder.